The present invention relates to mobile communication apparatuses used for cellular radio communication systems such as digital car telephones and portable telephones, especially mobile communication apparatuses which perform open-loop transmission power control and prevent deterioration of transmission diversity performance when performing VOX (Voice Operated Transmission) control.
In cellular radio communication systems such as car telephones and portable telephones, a CDMA (Code Division Multiple Access) system is known as one of the multiple access technologies which allow a plurality of mobile stations to carry out communications on a same frequency band simultaneously. There are also other systems known as the multiple access technologies such as FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access). The CDMA system has advantages over other technologies such as the ability to achieve high frequency utilization efficiency and accommodate more users.
The CDMA system uses user-specific spreading codes to distinguish a plurality of users who share the same frequency band and achieves multiple accesses through spread spectrum communications in which spreading codes are carried on an information signal which is transmitted with its spectrum spread over a sufficiently wide band relative to the original information bandwidth. The method of directly carrying spreading codes on the information signal is called xe2x80x9cdirect sequence.xe2x80x9d In the direct sequence CDMA system, signals from a plurality of mobile stations are multiplexed on a same frequency area and same time zone.
On the other hand, one of the radio communication systems using a same band on both transmitting and receiving sides is a TDD (Time Division Duplex) system. The TDD system is also called xe2x80x9cping-pong systemxe2x80x9d and is a system in which communications are carried out with a same radio frequency time-divided between transmission and reception. In contrast to the TDD system, there is an FDD (Frequency Division Duplex) system. The FDD system is a system in which communications are carried out using different frequencies between transmission and reception.
FIG. 5A shows a conceptual drawing of the TDD system and FIG. 5B shows a conceptual drawing of the FDD system. In FIG. 5A, at time T1, the base station transmits and the mobile station receives. At next time T2, the mobile station transmits and the base station receives. Communications using a single frequency band are achieved by repeating this.
In FIG. 5B, at frequency f1, the base station transmits and the mobile station receives, and at frequency f2, the mobile station transmits and the base station receives. The time unit for this transmission or reception is called a xe2x80x9cslot.xe2x80x9d
The direct sequence CDMA system has a so-called xe2x80x9cnear-far problemxe2x80x9d when a desired transmission station is far and a non-desired transmission station (interference station) is near. This problem is that the reception power of the signal from the interference station becomes greater than that from the desired transmission station which prevents the processing gain (spreading gain) alone from suppressing mutual correlation between spreading codes, which disables communication.
In a cellular radio communication system using the direct sequence CDMA system, it is therefore indispensable to perform transmission power control according to the state of each transmission path.
Another factor for deteriorating the line quality in terrestrial mobile communications is fading. As one of the fading countermeasures, a method of compensating variations of instantaneous values of the reception power by controlling the transmission power is proposed.
Theses xe2x80x9cStudy on Transmission Power Control in CDMA/TDD Transmissionsxe2x80x9d (Miya, Hayashi, Kato, 1994, IEICE spring B-418) and xe2x80x9cPOWER CONTROL IN PACKETS SWITCHED TIME DIVISION DUPLEX SEQUENCE SPREAD SPECTRUM COMMUNICATIONSxe2x80x9d (R. ESMAILZADEH, M.NAKAGAWA, A.KAJIWARA, proc. of VTC""92. pp.989-992, 1992) present transmission power control methods according to the CDMA/TDD system.
Fading has symmetry that variations are identical between transmission and reception if a same frequency band is used. Therefore, the CDMA/TDD system uses open-loop transmission power control which consists of detecting power of the reception signal, controlling the transmission situation by this and transmitting signals by determining the transmission power level according to the transmission situation. This allows the CDMA/TDD system to carry out more simple and high-speed and high-precision transmission power control than the FDD system.
Another effective fading countermeasure is space diversity technology. The space diversity technology receives radio signal using a plurality of antennas placed in different locations and combining and demodulating the received signals. Since fading variations of a received signal is non-correlative with each transmission path, the space diversity technology can reduce the probability of the reception power level of dropping due to fading, suppressing deterioration of the reception quality.
The TDD system also takes advantage of the transmission-reception symmetry in fading variations in the radio transmission path, places a plurality of antennas in the base station, allows the base station to carry out diversity reception, detect a transmission environment for each antenna and use the optimum antenna for transmission from the base station.
This allows space diversity to be performed even with a single antenna of the mobile station on the downlink, improving the line quality without expanding the hardware scale of the mobile station.
Thesis xe2x80x9cProposal for Base Station Transmission/Reception Diversity System in CDMA/TDD Transmissionxe2x80x9d (Miyaji, RCS 94-73, p.25-30, 1994-09) presents a transmission space diversity method according to the CDMA/TDD system.
VOX control in portable telephones, etc. is a technology that carries out transmissions only when there is voice to be transmitted and turns off transmission when there is no voice to be transmitted, contributing to power saving. In the CDMA system, the VOX control technology is an important technology in increasing the system capacity.
While the system capacity is fixed in the TDMA and FDMA, the system capacity for the CDMA is determined by the amount of interference from users who carry out communications simultaneously. This is called xe2x80x9csoft capacity.xe2x80x9d The system capacity in the CDMA system can be increased by reducing interference.
Stopping transmission by VOX reducing interference with other users, leading to an increase of the system capacity. For example, when the tone to silence ratio of all users is 50 percent, the number of users who are transmitting simultaneously is statistically a half, doubling the capacity in consequence.
However, if VOX control is performed in the CDMA/TDD system, there are slots which are not transmitted when there is no voice. When this happens, the receiving side cannot estimate the state of the transmission path from the received signal for the slots which are not transmitted. As a result, it is impossible to perform open-loop transmission power control and transmission space diversity, causing a problem of slowing down the fading variation speed that can be followed by control, deteriorating the speed performance.
The present invention has been implemented taking into account the actual situations described above and it is an objective of the present invention to provide a mobile communication apparatus that will not deteriorate the performance of open-loop transmission power control and transmission diversity even when VOX control is performed.
In order to achieve the aforementioned objective, the present invention performs VOX control such that when transmission on the base station transmitting side is OFF, the mobile station receives channels being always transmitted from the base station and performs open-loop transmission power control, and when transmission from the mobile station is OFF, it transmits dummy slots with small transmission power and the base station receives the dummy slots to carry out transmission space diversity.
The communication system of the present invention comprises a plurality of base stations and a plurality of mobile stations provided with communication means according to direct sequence CDMA/TDD (code division multiple access/time division duplex) systems as multiple access systems, with both the base stations and mobile stations equipped with a VOX function by which transmission is turned ON/OFF according to the presence/absence of transmission data during a communication. The base stations comprise a plurality of antennas, a reception diversity function to combine the signals received from a plurality of antennas above at a TDD reception time and a transmission diversity function that transmits at a TDD transmission time using the antenna which had the maximum reception power at the TDD reception time. The mobile stations comprise a function of transmitting dummy signals with smaller transmission power than normal transmission power when transmission is OFF.
Such a communication system performs VOX control that makes it possible to suppress deterioration of the performance of transmission space diversity at the base stations by using dummy signals even if transmission from the mobile stations is OFF.
Another communication system of the present invention has base stations comprising a common control channel transmission function for transmission/reception of control information to which a plurality of mobile stations are connected, and mobile stations comprising a function for detecting the power level of a received signal at a TDD reception time, a transmission power control function for controlling transmission power at a TDD transmission time using the received signal level above and a function for controlling transmission power by receiving the control channels above when transmission from the base stations is OFF.
This communication system performs VOX control that allows the mobile stations to receive control channels and use them to control transmission power even if transmission from the base stations is OFF, making it possible to reduce deterioration of the performance of open-loop transmission power control.